1. Field of the Invention
The invention relates to communication systems that use host signal processing (HSP), and in particular to interfaces and processes for maintaining a communication signal when a host computer skips a scheduled execution of a software portion of an HSP modem.
2. Description of Related Art
Host signal processing (HSP) uses a central processing unit (CPU) in a host computer to perform digital signal processing tasks that are more conventionally performed by hardware added to the host computer. For example, a conventional modem receives data from a host computer, converts the data to an analog signal in compliance with a communication protocol, and transmits the analog signal to a remote device. The conventional modem also receives an analog signal from the remote device, extracts data from the analog signal, and passes the data to the host computer. In an HSP modem, the host computer executes software that performs many of the conversions performed by a digital signal processor or other hardware in a conventional modem. Hardware in the HSP modem performs simple analog-to-digital and digital-to-analog conversions such as converting a received analog communication signal to a series of digital samples that represent amplitudes of the received signal. The host computer executes software that interprets the samples according to a communication protocol and derives the received data from the samples. The host computer also generates a series of output samples that represent amplitudes of a transmitted communication signal in compliance with the protocol, and hardware of the HSP modem converts the output samples into the transmitted signal. When compared to conventional modems, HSP modems have less complex (and less expensive) hardware because with HSP modems, the host computer performs many of the tasks performed by hardware in conventional modems. However, HSP modems use processing power of the host CPU, typically by periodically interrupting the host CPU.
Some communication protocols require that devices at both ends of a communication link transmit and receive constantly. The transmitted signal must be regular, even during a pause in data transmission, so that a receiving device recognizes that the transmitting device is still on the link. In such systems, an HSP modem generates regular interrupts to request new samples that are needed to maintain the transmitted signal. However, in some systems such as multi-tasking systems, the interrupts may be masked, or the host computer may be otherwise unable to execute software as required to maintain the transmitted signal. When the host computer is unable to respond, a lapse in transmission can occur, and the device at the other end of the communication link may disconnect or enter a retrain mode. This is unacceptable for modem users. Accordingly, an HSP communication system is needed which maintains a communication link even when a host computer is unable to respond to interrupts that for maintenance of a communication link.